Magnetic alloy and core

ABSTRACT

A magnetic alloy consisting essentially of less than 30 weight percent of cobalt, less than 30 weight percent of iron, less than 5 weight percent of a metal selected from the group consisting of copper and molybdenum, and the balance being essentially nickel and cores made therewith. The alloy has a Curie point greater than 620* C. and excellent rectangular hysteresis loop characteristics, such as, a Br/B10 value greater than about 90 percent.

United States Patent Inventors Sadami Tomita;

Hiroyuki Oouchi, both of Hitachi-shi, Japan Appl. No. 694,217 Filed Dec. 28, 1967 Patented Oct. 26, 1971 Assignee Hitachi, Ltd.

Tokyo-to, Japan Priority Dec. 28, 1966 Japan 41/8538! MAGNETIC ALLOY AND CORE 8 Claims, 7 Drawing Figs.

U.S. Cl 148/3155, 75/170,148/108, 148/121 Int. Cl 1101f 1/14, HOlf 3/02, C22c 19/00 Field 01 Search 75/123 K,

[56] References Cited UNITED STATES PATENTS 1,787,606 1/1931 White 75/123 1,841,097 1/1932 Elmen 75/123 1,715,541 6/1929 Elmen 75/123 1,715,542 6/1929 Elmen 75/123 2,158,132 5/1939 Legg 148/3155 X 2,783,170 2/1957 Littmann 148/3155 X Primary Examiner-L. Dewayne Rutledge Assistant Examiner-G. K. White Attorney-Craig, Antonelli, Stewart & Hill PATENTEI] BE 2 5 SHEET 2 [IF 2 HYSTERESIS CURVE INVENTORS SAMMI TOM/TA mow/ 1 (Ml/CHI BY $3 M ATTORNEY 5 MAGNETIC ALLOY AND CORE BACKGROUND OF THE INVENTION The present invention relates to a magnetic alloy and, more particularly, to a magnetic alloy having rectangular hysteresis loop characteristics to be used in saturable rector cores.

Saturable reactor cores to be employed in magnetic devices for automatic control or instrumentation, such as a magnetic amplifier or a magnetic multivibrator, are required to have a hysteresis loop characteristic which is as nearly rectangular as possible. On the other hand, in order to exhibit the most effective magnetic characteristics, anisotropic material has been formed into a toroidal core so that in the magnetic path there is no air gap that has a detrimental efi'ect on the rectangular hysteresis loop characteristics of the core. Material such as oriented SO-Permalloy or oriented Si-steel sheet, which has orientation achieved by rolling and annealing, is most preferably used as a toroidal core.

From the standpoint of use, however, toroidal cores have certain disadvantages in their difi'rcult winding operation and their poor space factor when assembled in a circuit device, as compared with the usual laminated cores punched out in E, I- type. Therefore, development of punched laminated cores having superior rectangular hysteresis loop characteristics has been desired for a long time.

Magnetic properties which are required for cores to be used in magnetic amplifier or magnetic multivibrators may be listed as follows:

1. Large magnetic flux density.

It a core material has a large magnetic flux density, a small sectional area of the core can be sufficient for a predetermined magnetic flux and the size of the device can be reduced greatly.

2. Small coercive force.

To have a large coercive force is not advantages for a core material since it requires a proportionally large exciting current. Furthermore, a large coercive force increases power loss in an alternating magnetic field and results in a temperature rise of the core.

3. Rectangular hysteresis loop characteristics.

Core material for magnetic amplifiers or magnetic multivibrators is required to have such hysteresis loop characteristics that the rise of magnetization curve is sharp and the approaching portions of the curve to the saturation of magnetization are fiat.

Generally speaking, a saturable reactor is required to have a rectangular ratio Br/Bm or Bk/Bm greater than 0.9, where Br is the residual magnetic flux density, Bk is the magnetic flux density at the knee point of the magnetization curve, and Bm is the maximum magnetic flux density.

4. Small temperature coefficient of magnetic properties.

A metallic magnetic substance has a distinguished merit with respect to small temperature coefficient of magnetic properties as compared with ferrite (an oxide magnetic substance). Although even metallic substances have different temperature coefficients according to the compositions of the alloys thereof, the change of magnetic properties of cores should be as small as is practicably possible when they are used as circuit elements for automatic control or instrumentation.

5. Large electrical resistivity.

When the core is placed in an alternating magnetization field, the resulting eddy current loss is inversely proportional to the electrical resistivity of the core material. Therefore, its electrical resistivity should be as large as is practically possible.

6. Small change in magnetic properties owing to external stress.

Although core material of both a small coercive force and rectangular hysteresis loop characteristics tends to change its magnetic properties owing to an external stress, as small a change in magnetic properties as possible is desirable from the practical point of view.

SUMMARY OF THE INVENTION Accordingly, it is the main object of the present invention to provide a new magnetic alloy having superior rectangular hysteresis loop characteristics.

Another object of the invention is to provide a core material of rectangular hysteresis loop characteristics which substantially satisfies most of the above-mentioned requirements.

Still another object of the invention is to provide a core material of the above character which essentially satisfies all of the six requirements above-mentioned.

Still a further object of the present invention is the provision of a punched core element of the above character which has the advantage of an easy winding operation.

According to the invention, there is provided a magnetic alloy consisting essentially of a FE-Co-Ni system alloy which less than 5 weight percent of M0 or Cu is and which has a Curie point higher than 620 C. This alloy is effectively influenced by magnetic annealing treatments.

These and other objects and features of the invention will be apparent from the following drawings and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows the Curie points of Fe-Co-Ni system alloys plotted as a Fe, Co, Ni ternary phase diagram;

FIG. 2 shows spontaneous magnetizations of the same alloys plotted similarly to FIG. 1;

FIG. 3 shows coercive forces of the same alloys plotted similarly to FIG. 1;

FIG. 4 is a plan view showing a U-shaped core element made of a magnetic alloy according to the invention;

FIG. 5 is an explanatory view to show a magnetic annealing treatment of a core constructed by lamination of the core elements shown in FIG. 4;

FIG. 5 is an explanatory view to show a magnetic annealing treatment of a core constructed by lamination of the core elements shown in FIG. 4;

FIG. 6 is a perspective view ofa core constructed by lamination ofthe U-shaped core elements according to the invention; and

FIG. 7 shows a part of a magnetization curve to explain the definition of the rectangular ratio Br/Bm or Bk/Bm.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As noted above, the applicants have provided Fe-Co-ni system alloys to develop a core material of rectangular hysteresis characteristics which satisfies the six requirements previously pointed out, have adopted the magnetic annealing treatment to achieve its rectangular hysteresis characteristics, and have studied the shapes ofa core element about the same, such as U shape, ring shape and the like. Next will be explained the magnetic properties of Fe-Co-Ni alloys in reference to ternary phase diagrams as a basis for this invention.

FIGS. 1, 2 and 3 show the Curie points, spontaneous magnetization (in Gauss) and coercive forces (in Oersted), respectively, of Fe-Co-Ni system alloys. These drawings are derived from the book Ferromagnetism," written by R. M. Bozorth and published by D. Van Nostrand Co. in [951. As

will be clear from the drawings, if the amount of Ni is greater than 50 weight percent, the Curie point increases as the amount of Co increases. On the other hand the Curie point decreases rapidly as the amount of Fe increases beyond 30 weight percent. Where Co 30% and Fe l0% the spontaneous magnetization of the alloys decreases to a large extent and where Co 30% and FE l0%, the coercive forces of the alloys are large. It will be apparent that the closer to I that the atomic ratio of Ni and Co plus Fe is, the larger is the induced magnetic anisotropy due to the magnetic annealing treatment, and that the higher the Curie point of the alloy, the lower is the temperature coefficient of the magnetic properties thereof.

In order to satisfy the objects of the invention, possible ranges of compositions of alloy will be limited to such Fe-Co- Ni system alloys that Fe plus Co 50%, 10% Fe 30 and the amount of Ni is not excessively great, preferably less than EXAMPLE 1 Fifteen different alloys (Sample Nos. l-l5,shown in table l) were produced by melting in a vacuum. Electrolytic Fe, Co, and Ni of very high purity were used in preparing the alloys t r I t 5 80 weight percent in view of its practical amount insofar as and as for molybdenum and Copper, element metals of very spontaneous magnetization 1s concerned. The amounts of Co and Fe cement d H l d high purity were also used.

5 o F percent An ingot was made into a billet of a thickness of mm. by Increased amounts of Co m such system alloys hot working which was in turn intermediately annealed and zfii s g g z 9 zf f lmpwvdemem of 10 rolled into a plate of final thickness of 0.3 mm. by cold rolling. u m gne P F w let e increase amount Subsequently, ring samples of an outer diameter of 45 mm. of Co not only makes its coercive force large, but also its coefand an inner diameter of 33 mm were punched out from the figl gzg tg sgzg z szzg o; large l t m 59 1 plate and were utilized for experimentation. In order to reduce h f b l u Ema S r655 the strain caused by working, the samples were annealed at a geeflcaenllpunt o co a t should be limited to less than 30 weight 15 temperature greater than in a hydrogen gas mos here so as to revent oxidation of the sam les or con- On the other hand, if the amount of cobalt in the alloy is too tamizafion of the sgmples by the atmosphere sfbsequemly small the cum pomt f the spontanecfus magnetfzatmn of the samples were subjected to a magnetic annealing treatthe alloy decrease considerably and superior magnetic propermm First, a ring core was wound with a m produce a cannot be p h h {iccomphsh the foregoing oblects magnetic field therein and was heated again up to 700C. The of the Present mvehuoht ll hecessflry that the amount of core was cooled gradually under the magnetic field from 700 cobalt be more than least 5 welght Percent Preferfed C. to 400 C., wherein the cooling rate is about C. lhr. amounts thel'eof am wh-hlh the range of fmm 7 to 21 welght 200 C. /hr. and the strength of the magnetic field is about 2-8 P oersteds.

From the foregoing observations, it will be clear that desira- 25 T i 1 Shows gjgg g fi density 310 rectangular g: S i i g y; m g g sz g ratios Br/BlO and nos/B10, coercive force 11, the Curie 0 l 0 point Tc, electrical resistivity p(p..(l.-cm.) cm.) and tempera- The 5e alloys, however, have a disadvantage in that. because of ture coefficient of the magnetic flux density a of each samtherr small electrical resistivity, they cannot satisfy the six le hen itwas annealed at l,l00C. for 2 hours, heated again requirements previously p n n y cannot be up to 700 C. and kept there for one-half hour and thereafter suitably used in magnetic devices such as a magnetic amplifier ooled under a magnetic field of 5 oersteds to 400 C. at a or a multivibrator where a small power loss is required. cooling rate of 100 C. lhr. In the table, a is an average temln order to solve this problem of small electrical resistivity, e ature oeffi ient between 20 C, and +30 C., and B10, many studies and experiments were carried out and the appli- Br/Bl0,BO. 5/Bl0 are determined according to an indication cants found that the addition of a proper amount of either of magnetization values as shown in FIG. 7. The superiority of molybdenum or copper is effective for increasing the electrirectangular hysteresis characteristics can be seen by the excal resistivity of the alloy. Furthermore, the addition of M0 or tent of closeness of the values of Br/BIO and B0. 5 [B10 to Cu has been found to be effective for improving the rectangu- ,l00 percent.

TABLE 1 Sample Chemical compositions B10 Br/BIO 130.5113 10 He To No. in Weight percent Org.) (percent) (percent) (00.) t C.)

1 7Co-25FeNi 12. 7- 86.6 90. 4 0. 041 080 7Co-23Fe-2Mo-Ni 12.1 110.6 93.8 0.041 630 a. 7Co-23Fe-2Cu-N1 12.2 03. 2 s8. 0 0. 030 640 4.. ZCO-21Fe4M0-Ni 11.0 86. (i 81.3 0.105 580 5.. .Co-21Fe-4Cu-N1 11.3 75. 5 217.5 0.066 600 0.. 14Co-26Fe-Nl 13.8 88.11 02. 2 0. 03s 6210 r 14Co-24Fe-2M0-N1 12.8 01.3 05. 0 0. 033 640 8.. 14C0-24Fe-2Cl1-N1 13. 0 112. 4 as. 1 0. 031 650 9... 14Co-22Fe-4Mo-Ni 11.4 80. 7 s5. 4 0. 002 600 10... 14Co22Fe4Cu-Nl 11. 5 as. 3 02. 3 0. 043 620 11 2lC0-26Fe Nl 14. 0 37.8 02.9 0. 040 710 12 21C0-24Fe-2M0-Nl l3. 7 819. 3 .13. 8 0. 030 670 21Co-24Fe-2Cu-N1 13.9 01.0 05.2 0.025 670 21co-22Fe-1Mo-Ni 12.6 35.3 211.1 0. 040 630 21Co-22Fe-4Cu-N1 12.0 85.11 :10. 7 0. 035 040 I lar hysteresis characteristics thereof. One of the objects of the From table I it will be noted that although F e-Co-Ni ternary invention is not only to improve the magnetic properties of an alloy Sample NOS. 1, 6 and l l h ve higher ri points and alloy, such as the coercive force and rectangular ratio, but also relatively g magnetic Properties, their electrical resistivi' to increase the electrical i i i th f; hi can b ties are considerably small. Asmall electrical resistivity causes tained by the addition of less than 5 weight percent of either a iafge y a Core which suhlefled molybdenum or copper to the base alloy. Furthermore, if core ham/e maghehzauohmaterial according to the invention is employed, a core to be contrary auoys much used in a magnetic amplifier or a multivibrator can be formed coma"! a p p 1 mm! f molybdenum or copper, other in a uflshapey which Still retains the desired rectangmm. than Fe, Co, and N1, have a large electrical resistivity. Except hysteresis loop characteristics thereby greatly improving the Samples 2 4 5 and the auys a space factor and making the winding operation quite simple h'gher than 620 and have Superior magnenc prowl-ms and easy In order to improve the rectangular hystersis characteristics, the alloy should be effectively influenced by a mag- 7 netic annealing treatment so as to have strong magnetic EXAMPLES OF THE NVENTION anisotropy. The inventors have found that the amount of in- Th f n c I I 11 f fluence of the magnetic annealing treatment on anfallgy has an e o owing examp es are given mere y as 1 ustratlve 0 important relationship with the Curie point 0 t e a oy; the present invention and are not to be considered as limiting. generally speaking, alloys of high Curie points show superior Unless otherwise noted, the percentages therein are by weight. 7 magnetic properties.

In the Sample Nos. 4, 5 and 9, the coercive force is large and both Br/BlO and BUS/B10 are small although these samples have compositions falling within ranges that Fe 30 Co 30 M or Cu 4 and the balance is Ni. Their Curie 3 200 C./hr. 11.6 88.6 0.045 [00 C./hr. l2.l 93.2 0.027 50c./hr. l2.l 93.8 0.026

points tend to be lower than 620 C. since, as will be understood from the F105. 1, a small percentage of cobalt tends These Samples were fi t maintained at 700 C f 30 to make the Curie P lower and, furthermore the addition minutes and then they were cooled at various cooling rates as of molybdenum pp functions to lower the Curie p shown in table 3 under a magnetic field of 5 oersteds until Accordingly, the amount of molybdenum or copper to be 400 was reached added should be preferably chosen such that the Curie point EXAMPLE Iv of the ig may be g z f 5 z Table 4 shows the magnetic properties of Fe Co Ni base al ture coe icient a o e o eo-Ni- 0 or eoi- Cu System alloys 14 for the average value loys when 2 weight percent of copper and trans1t1on metals between 20 C. and +30 C. and is about one-half of those of Such as molybdenum ehromlum e' were aeded the usual known materials used as toroidal cores, such as 15 ge E tf a weredformed i g t g 't e fgorg oriented SO-Permallo 50 ercent Ni-Fe allo 01 5.5X o an were annea e a a s 10 Supermalloy perc ent Mo -79 perce nt t l i -Fe alloy; T f z i g g r z e p werel fi a =(6.0 Xl0)]. On the other hand, it is noted amon the mam ame a or es an en were e00 e 3 samples that those having higher Curie points tend to i therate of 50 C. per hour under a magnetic field of5 oersteds Small absolute values anr until a temperature of 400 C. was reached.

The samples Nos. 2, 3, 7, 8,l0,12,l3,14 and 15 satisfy the TABLE 4 conditions that B10 Z115 kg. (k gauss), Br/Bl0 85 Hc 0.05 0e, l p 30p.(1cm. and a 2.5Xl0" and, there,- C H H fore, are found to be quite suitable for a core material of rectangular hysteresis loop characteristics to be used in a magsample (welsh! percent) BIO we) New (m (0:) netic amplifier, a magnetic multivibrator and the like. Though l6 7CO 2lFc 2M Ni H l 86 i 0 039 table 1 shows only annealing conditions at the high temperal7 7C 2|Fe 2Cu Ni tures prior to the magnetic field cooling treatment, it goes 1s 7Co-2lFe-2Cr-Ni 11.0 66.6 0.066 without saying that the samples were in turn subjected to the magnetic field cooling so as to impart the anisotropy and the rectangular hysteresis loop characteristics required for the A b l f t bl 4 th ff t f h dd? f saturable reactor core to the allo s. 5 W1 6 c ear Tom a e 1 e 6 cc 5 0 e a 1 y these metals on magnetic properties, especially rectangular EXAMPLE n hysteresis characteristics, are quite different, although all of TWO kinds 0f alloys were examined 85 K0 the influence Of an- [hem were expected to improve the magnetic properties as nealing temperatures, Show" in table The Samples are well as to increase the electrical resistivity. For Fe-Co-Ni base 3' composed Oflhe Sample N051 and 301mb? alloys, copper and molybdenum were found to be the most preferable additive elements not only to increase the electrical TABLE 2 resistivity but to improve the rectangular hysteresis characteristics by a magnetic annealing treatment. Annealing Condition B10 Hag.) Br/BIO (1) HctOe) EXAMPLE v Sample 2 A core was made by lamination of U-shaped core elements, as shown in FIG. 4, which were punched out from a plate or i fiz s'gigzfz 23': 3'82: plates composed of the Sample No. 3 or No. 10 which disj l ham 0:040 played excellent characteristics as shown in table I.The core 1.2o0c.x2 hours 11.7 114.5 0.031 element in FIG. 4 has a thickness of about 0.3 mm., I of four 7 g" different lengths as shown in table 5, 1 of 10 mm., 1;, of 20 Z'Z S L9 917 04055 mm. and I, of 5 mm. After being punched, the core element 90oc.x2 hours 11.9 93.4 0.045 was annealed at a high temperature and subsequently it was annealed under a magnetic field from 700' C. to 400-20 H82. 22:: 8:2; 400at a rate of 100 C./hr. The magnetic annealing was done me hours 316 M20 as shown in FIG. 5, wherein l is a coil, 2 is a U-shaped laminated core and 3 is a ceramic cylinder on which the coil 1 EXAMPLE m is wound. Legs of each core element were inserted into the ceramic cylinders from opposite sides alternatively in order to The influence of the cooling rate under a magnetic field on form a flame4ype com 400C two kinds of alloys composed ofSample Nos. 2 and 3 were ex- A re t a a lied to the oil to produce a magnetic amined as shown in table 3. field of 4 oersteds therein. After completion of the magnetic annealing treatment, the core elements were taken out and 30 TABLE of them were assembled as a unit to form a core by lamination in which the open ends of legs of each core element were piled Sample 58:23: 810 (kw Br/Bw (m Heme) on the closed ends of legs of other core elements as shown in FIG 6, wherein 4 are leads of soleno dal co1ls 5, 6 1s a j 2 200 an". H s 7 0-055 laminated U-shaped core and 7 are insulating plate members.

2 ohm 904 M40 The magnetic properties of these laminated cores are shown 2 50 cm. 11,7 90.5 0.032 in table 5.

TABLE 5 Length of legs =50 =55 =l00 Sample No Magnetic properties mm. mm. mm. mm.

a B10 (kg.) 12.0 12.1 12.1 12.1 Br/Blo (percent) 83. 0 88. 5 80. 7 89. 4 B0.5/B 10 (percent) 90. 4 93. 7 U5. 5 JG. 0 He (00.) 0. 036 0. 037 0. 037 0. 038

, TABLE Continued Length of legs Sample No. Magnetic properties mm. mm. mm. mm.

BIO (kg) 11.3 11.4 11.4 11.4

- Br/BIO (percent) 80. 2 86. 5 87. 3 87. 0

B0.5/B 10 (percent).-- 88. l 92. 9 93. 2 94. 0

As will be understood by a comparison of table I with table 10 l. A magnetic alloy having a Curie point greater than about 5, the rectangular hysteresis characteristics of the laminated 620 C. and a rectangular hysteresis loop characteristic U-shaped core are somewhat inferior to those of the ring core. This is caused by small gaps formed about the opposite ends of the legs of the laminated core by alternative lamination of U- shaped core elements in opposite directions. If 1 of the laminated core is greater than is greater than 55 mm., however, its rectangular hysteresis characteristics are such that Br/B10 percent and BO.S/B10 9O percent, these being suitable for a core for a magnetic amplifier or a magnetic multivibrator. It will be also noted in table 5 that as 1 increases, both its rectangular ratio and its coercive force tend to increase. Furthermore, it is apparent that a ratio of the length I of the legs to length 1 of the yoke should be at least 2.5, most preferably within a range of from three to five.

As compared with the usual toroidal cores, a laminated core according to the present invention has a further advantage with respect to its resistance to deterioration of magnetic properties by externally applied stress or shock. Asan experi ment, such an external force as having an amplitude of 3 g, where g is the acceleration of gravity, was applied for a whole day and night to both a U-shaped core according to the invention and a toroidal core of oriented SO-Permalloy having a thickness of 0.1 mm. Although the rectangular ratio of the toroidal core decreased by 5-7 percent, no deterioration of any magnetic properties was observed in regard to the core according to the present invention.

While the invention has been described in connection with certain modifications thereof, it should be understood that further modifications may now suggest themselves to those skilled in the art, and it is intended to cover such modifications as fall within the scope of the appended claims without departing from the spirit and scope of the invention.

We claim:

represented by a Br/B ratio greater than 0.9 imparted by a magnetic cooling treatment, consisting essentially of from about 7 to 21 percent by weight of cobalt, from about 20 to 25 percent by weight of iron, from about 1.5 to 4 percent by weight of at least one additive element selected from the group consisting of molybdenum and copper, and the balance being nickel.

2. A magnetic alloy according to claim 1, wherein said additive element is copper.

3. A magnetic alloy according to claim 1, wherein said additive element is molybdenum.

4. A magnetic sheet material used for a saturable reactor core which comprises a magnetic alloy having rectangular hysteresis loop characteristics represented by a Br/B value greater than about 90 percent, imparted by a magnetic cooling treatment, a sharply rising magnetization curve characteristic, and a Curie point greater than about 620 C., consisting essentially of from about 7 to 21 percent by weight of cobalt, from about 20 to 25 percent by weight of iron, from about 1.5 to 4 percent by weight of an element selected from the group consisting of molybdenum and copper, and the balance being nickel.

5. A magnetic sheet material according to claim 4, in which the sheet material is U-shaped and the ratio of the length l of the legs to the length 1 of the yoke is at least 2.5.

6. A magnetic sheet material according to claim 4, in which the magnetic alloy has an electric resistivity greater than about BOO-cm.

7. A magnetic alloy sheet material according to claim 4, wherein said additive element is copper.

8. A magnetic alloy sheet material according to claim 4, wherein said additive element is mglyl denum.

UNITED STATES PATENT OFFICE 1 CERTIFICATE OF CORRECTION Patent No. 910 D t d October 26, 197 1 lnventofls) Sadarni TOIVIITA, HiroyukiOOUCHI It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In Column 2, line 16, change 'FE-Co-Ni" to read Fe-CO-Ni line 46, change "Fe-Co-ni" to read Fe-Co-Ni line 69, change "FIE" to read Fe In Column 4, line 27, change n-cm. )cm. to read (gJZ-cm.)

I u a 1| 14! "'4 n Column 5, hne 13, change 10 to read 10 line 17, change "10' "1:o read 10" line 18, change "10 to read 10 line 23, change "y to J line 23, change "10 to 10 In Column 6, line 52, delete "400-20",

"line 58, change "flame-type" to frame-type In Column 8, line 39, change J2 -crn" to read pu l-cm Signed and sealed this 5th day of December 1972.

(SEAL) Attest:

EDWARD M.FLETCI-IER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents )RM Pro-1050 (10-69) USCOMMDC 60375-p5g \1 U S GUVERNMENT PRINHNQ OFFICE I959 036tw330 

2. A magnetic alloy according to claim 1, wherein said additive element is copper.
 3. A magnetic alloy according to claim 1, wherein said additive element is molybdenum.
 4. A magnetic sheet material used for a saturable reactor core which comprises a magnetic alloy having rectangular hysteresis loop characteristics represented by a Br/B10 value greater than about 90 percent, imparted by a magnetic cooling treatment, a sharply rising magnetization curve characteristic, and a Curie point greater than about 620* C., consisting essentially of from about 7 to 21 percent by weight of cobalt, from about 20 to 25 percent by weight of iron, from about 1.5 to 4 percent by weight of an element sElected from the group consisting of molybdenum and copper, and the balance being nickel.
 5. A magnetic sheet material according to claim 4, in which the sheet material is U-shaped and the ratio of the length l of the legs to the length l3 of the yoke is at least 2.5.
 6. A magnetic sheet material according to claim 4, in which the magnetic alloy has an electric resistivity greater than about 30 Omega -cm.
 7. A magnetic alloy sheet material according to claim 4, wherein said additive element is copper.
 8. A magnetic alloy sheet material according to claim 4, wherein said additive element is molybdenum. 